Fuel economy improves if an internal combustion engine is run at a lean air-fuel ratio, but during lean running, the NOx in the exhaust gas increases. Therefore, a NOx storage catalyst is used as exhaust catalyst for an engine which performs lean air-fuel ratio running.
The NOx storage catalyst traps and stores NOx contained in the exhaust gas when the engine is running at a lean air-fuel ratio. When the engine is run at a rich or stoichiometric air-fuel ratio, the stored NOx is reduced by components in the exhaust gas such as HC (hydrocarbons) and CO.
However, SOx (sulfur oxides) in the exhaust gas are also trapped and stored by this NOx storage catalyst. Therefore, the ability of the NOx storage catalyst to trap NOx falls if the amount of stored SOx increases (SOx poisoning).
In Tokkai Hei 10-54274 published by the Japanese Patent Office in 1998, when the amount of SOx stored in the NOx storage catalyst increases and NOx trapping performance declines, a misfire is caused at a lean air-fuel ratio for a predetermined time, and unburnt fuel is supplied to the NOx storage catalyst. By burning this unburnt fuel on the catalyst or by retarding ignition time to increase the exhaust gas temperature, the catalyst temperature is increased, and the stored SOx is thereby discharged.